U.S. patent number 6,707,602 [Application Number 10/408,075] was granted by the patent office on 2004-03-16 for reflective spectral filtering of high power extreme ultra-violet radiation.
This patent grant is currently assigned to Intee Corporation. Invention is credited to Michael Goldstein.
United States Patent |
6,707,602 |
Goldstein |
March 16, 2004 |
**Please see images for:
( Certificate of Correction ) ** |
Reflective spectral filtering of high power extreme ultra-violet
radiation
Abstract
In one embodiment of the invention, a grating structure etched
on a mirror substrate has a grating period causing diffracting, out
of an optical path, a first incident radiation within a first band
around a first wavelength. A multi-layer coating deposited on the
grating structure reflects the first incident radiation, in the
optical path, within the first band and a second incident radiation
within a second band around a second wavelength. In another
embodiment, a first multi-layer coating deposited on a mirror
substrate reflects a first incident radiation within a first band
around a first wavelength and a second incident radiation, in an
optical path, within a second band around a second wavelength. A
grating structure is deposited on the first multi-layer coating.
The grating structure is etched to have a grating period causing
diffracting, out of the optical path, the second incident radiation
within the second band.
Inventors: |
Goldstein; Michael (Ridgefield,
CT) |
Assignee: |
Intee Corporation (Santa Clara,
CA)
|
Family
ID: |
25509562 |
Appl.
No.: |
10/408,075 |
Filed: |
April 3, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
965170 |
Sep 27, 2001 |
6577442 |
|
|
|
Current U.S.
Class: |
359/361; 359/359;
359/360; 359/565; 359/566 |
Current CPC
Class: |
G03F
7/70158 (20130101); G03F 7/70191 (20130101); G03F
7/702 (20130101); G03F 7/70575 (20130101) |
Current International
Class: |
G03F
7/20 (20060101); G02B 005/20 () |
Field of
Search: |
;359/361,359,360,565,566,572,571,576,584,585,586,350,351,355,858
;378/34,35 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Sikder; Mohammad
Attorney, Agent or Firm: Blakely, Sokoloff, Taylor &
Zafman LLP
Parent Case Text
CROSS-REFERENCES TO RELATED APPLICATIONS
This is a Divisional Application of U.S. patent application Ser.
No. 09/965,170, filed Sep. 27, 2001 now U.S. Pat. No. 6,577,442.
This Divisional Application claims the benefit of the U.S. patent
application Ser. No. 09/965,170.
Claims
What is claimed is:
1. An apparatus comprising: a first multi-layer coating deposited
on a mirror substrate to reflect a first incident radiation, in an
optical path, within a first band around a first wavelength and a
second incident radiation within a second band around a second
wavelength; and a grating structure deposited on the first
multi-layer coating, the grating structure being etched to have a
grating period causing diffracting, out of the optical path, the
second incident radiation within the second band.
2. The apparatus of claim 1 wherein the grating structure
comprises: a plurality of ridges spaced at the grating period, the
ridges having a ridge width and height, the ridge width being
approximately proportionally to the grating period with a first
proportionality constant, the ridge height being approximately
proportionally to the grating period with a second proportionality
constant.
3. The apparatus of claim 2 wherein each of the ridges comprises:
one of a metal spacer, a second multi-layer coating, and a
combination of the metal spacer and the second multi-layer coating,
the metal spacer providing grating spacing, the second multi-layer
coating reflecting the first incident radiation within the first
band and the second incident radiation within the second band.
4. The apparatus of claim 1 further comprises: a stop layer
deposited between the grating structure and the first multi-layer
coating to protect the first multi-layer coating during etching the
grating structure.
5. The apparatus of claim 1 wherein the first wavelength is at
approximately 13.4 nm.
6. The apparatus of claim 3 wherein each of the first and second
multi-layer coatings comprises a plurality of layers of first and
second materials having one of high and low atomic numbers,
respectively, and high and low densities of charge carriers,
respectively.
7. The apparatus of claim 6 wherein the first material is
molybdenum (Mo).
8. The apparatus of claim 6 wherein the second material is one of
silicon (Si) and beryllium (Be).
9. The apparatus of claim 6 wherein each of the first and second
multi-layer coatings further comprises: a plurality of layers of a
compound interspersed within the plurality of the first and second
materials.
10. The apparatus of claim 9 wherein the compound is silicon
carbide (SiC).
11. A system comprising: a mirror to reflect an extreme ultra
violet (EUV) radiation; a baffle having an opening positioned to
stop diffracted radiation rays and allowing actinic radiation rays
to pass through the opening; and a reflective spectral filter
positioned to generate the diffracted radiation rays and the
actinic radiation rays from the reflected EUV radiation, the
reflective spectral filter comprising: a first multi-layer coating
deposited on a mirror substrate, the first multi-layer coating
reflecting a first incident radiation, in an optical path, within a
first band around a first wavelength and a second incident
radiation within a second band around a second wavelength, and a
grating structure deposited on the first multi-layer coating, the
grating structure being etched to have a grating period causing
diffracting, out of the optical path, the second incident radiation
within the second band.
12. The system of claim 11 wherein the grating structure comprises:
a plurality of ridges spaced at the grating period, the ridges
having a ridge width and height, the ridge width being
approximately proportionally to the grating period with a first
proportionality constant, the ridge height being approximately
proportionally to the grating period with a second proportionality
constant.
13. The system of claim 12 wherein each of the ridges comprises:
one of a metal spacer, a second multi-layer coating, and a
combination of the metal spacer and the second multi-layer coating,
the metal spacer providing grating spacing, the second multi-layer
coating reflecting the first incident radiation within the first
band and the second incident radiation within the second band.
14. The system of claim 11 wherein the reflective spectral filter
further comprises: a stop layer deposited between the grating
structure and the first multi-layer coating to protect the first
multi-layer coating during etching the grating structure.
15. The system of claim 11 wherein the first wavelength is at
approximately 13.4 nm.
16. The system of claim 13 wherein each of the first and second
multi-layer coatings comprises a plurality of layers of first and
second materials having one of high and low atomic numbers,
respectively, and high and low densities of charge carriers,
respectively.
17. The system of claim 16 wherein the first material is molybdenum
(Mo).
18. The system of claim 16 wherein the second material is one of
silicon (Si) and beryllium (Be).
19. The system of claim 16 wherein each of the first and second
multi-layer coatings further comprises: a plurality of layers of a
compound interspersed within the plurality of the first and second
materials.
20. The system of claim 19 wherein the compound is silicon carbide
(SiC).
Description
BACKGROUND
1. Field of the Invention
This invention relates to lithography. In particular, the invention
relates to extreme ultra-violet (EUV) lithography.
2. Description of Related Art
Extreme ultraviolet (EUV) lithography is an imaging technology with
higher resolution capabilities than are available from longer
wavelength exposure tools. The very short exposure wavelength
requires an all reflective lens system because refractive materials
are not sufficiently transparent. The EUV technique bounces EUV
photons off a system of mirrors, including a mask made of
reflective materials, that is ultimately focused on a resist-coated
silicon wafer. By doing so, EUV systems can pattern features
smaller than 0.05 micrometer.
Present EUV radiation sources have a very broad-band emission
spectrum. For example, Xe laser pulsed plasma (LPP-Xe) sources have
in excess of 40% of their radiation at wavelengths longer than 125
nanometers (nm) and approximately 60% of the energy is at
wavelengths that are longer than 17 nm. EUV lithography tools for
high volume manufacturing may require about 13.4 nm +/-1%
wavelength radiation. Power levels are expected to be in the range
between 50 and 100 Watts. Thus, kilowatts of energy potentially
need to be filtered from the source spectra. Otherwise, it would
unreasonably distort the mask and mirrors used in EUV imaging
tools.
Existing techniques include use of ultra-thin transmission filters
based on coating of a membrane on support structures. This
technique has low efficiency, typically passing only about 50% of
the desired wavelength. In addition, the membrane filters are easy
to rupture at high power levels because of absorption. Another
technique uses mono-chrometer and diffraction of the actinic 13.4
nm light. This technique is substantially less efficient and is
generally used with only synchroton sources. Yet, another technique
uses cooled Mo/Si multi-layer coated mirrors. This technique
operates only through absorption and selective reflection, leading
to problems in heating and a lack of long wavelength filtering.
Therefore, there is a need to have an efficient technique for
spectral filtering of high power EUV radiation.
BRIEF DESCRIPTION OF THE DRAWINGS
The features and advantages of the present invention will become
apparent from the following detailed description of the present
invention in which:
FIG. 1 is a diagram illustrating a system using a reflective
spectral filter in which one embodiment of the invention can be
practiced.
FIG. 2 is a diagram illustrating a process to fabricate the
reflective spectral filter shown in FIG. 1 according to one
embodiment of the invention.
FIG. 3 is a diagram illustrating a process to fabricate a
reflective spectral filter shown in FIG. 1 according to another
embodiment of the invention.
FIG. 4 is a diagram illustrating reflectance and absorption spectra
for the multi-layer coatings according to another embodiment of the
invention.
FIG. 5 is a diagram illustrating a system using a transmitting
spectral filter in which one embodiment of the invention can be
practiced.
FIG. 6 is a diagram illustrating a process to fabricate the
transmitting spectral filter shown in FIG. 5 according to one
embodiment of the invention.
DESCRIPTION OF THE INVENTION
The present invention is a technique for efficient spectral
filtering of EUV radiation. The spectral filter includes a grating
structure and a multi-layer coating. The multi-layer coating is
designed to reflect a narrow band around a short wavelength (e.g.,
13.4 nm) and also reflects long wavelengths (e.g., greater than
approximately 60 nm). The grating structure has a grating period
responsive to the long wavelength band. The multi-layer coating is
formed by layers of materials having high and low atomic numbers
(e.g., Mo and Si) interspersed by layers of a compound (e.g., SiC).
Regions of the longer wavelength rays are removed from the optical
path by diffraction. The actinic rays have a wavelength much
shorter than the grating period and undergoes simple reflection.
The combined properties of the multi-layer coating and the grating
structure generate the desired spectral characteristics.
In the following description, for purposes of explanation, numerous
details are set forth in order to provide a thorough understanding
of the present invention. However, it will be apparent to one
skilled in the art that these specific details are not required in
order to practice the present invention. In other instances,
well-known structures are shown in block diagram form in order not
to obscure the present invention.
FIG. 1 is a diagram illustrating a system using a reflective
spectral filter in which one embodiment of the invention can be
practiced. The system 100 includes an EUV radiation source 110, a
condenser mirror 120, a reflective spectral filter 130, and a
baffle 140. The system 100 is typically used in EUV
lithography.
The EUV radiation source 110 is a laser pulsed plasma source. A
Xenon (Xe) gas jet is hit with a high power laser pulse. Ionization
and recombination generates spectral lines ranging from X-ray to
Infra-red. As is known by one skilled in the art, any other methods
to provide the EUV radiation source can be employed.
The condenser mirror 120 is an optical subsystem to collect the EUV
radiation from the EUV radiation source 110.
The reflective spectral filter 130 receives the source radiation as
provided by the condenser mirror 120 and to reflect actinic rays
160 and diffracted rays 150. In one embodiment, the actinic rays
are radiation at 13.4 nm and the diffracted rays 150 are at
wavelengths longer than the actinic wavelength (e.g., 60 nm). The
reflective spectral filter 130 will be described in FIGS. 2 and
3.
The baffle 140 is a metal plate to stop the diffracted rays 150.
The baffle 140 has a hole 145 aligned with the principal optical
path of the spectral filter 130. The actinic rays 160 are on the
optical path and go through the hole 145.
FIG. 2 is a diagram illustrating a process 200 to fabricate the
reflective spectral filter 130 shown in FIG. 1 according to one
embodiment of the invention.
The process 200 starts with polishing a blank mirror substrate 210.
The mirror substrate 210 may have a flat or curved surface. The
mirror substrate 210 may be made of Zerodur, ULE, or composites
with low coefficients of thermal expansion that can be polished
well. Then, a relief material or a photo-resist layer 220 is
deposited on the mirror substrate 210.
Next, the relief layer 220 is lithographically patterned and etched
to form a grating structure 220. The grating structure 220 has a
plurality of ridges spaced at a grating period T. It is noted that
although the preferred embodiment has a periodic pattern of ridges,
it is contemplated that non-periodic pattern may also be used. The
ridge shape is not restricted to the rectangular cross-section
shown and could be triangular or other possibilities. The grating
structure 220 may have a one-dimensional layout or a
two-dimensional layout. The layout pattern 250 shows a
representative two-dimensional layout having ridges 252 and grooves
254. The grating period T is selected to be responsive to a band
around a design filter wavelength and selected harmonics. The
grating period T is selected to cause diffracting, out of an
optical path, an incident radiation within the band of the design
filter wavelength. The design filter wavelength may be 60 nm
corresponding to when coating reflectivity begins to increase, 140
nm corresponding to significant emission spectra of the EUV source,
or other possibilities. The ridges have a ridge width W and height
H. The ridge width W is typically less than the grating period T.
In one embodiment, the ridge width W is approximately
proportionally to the grating period T with a proportionality
constant .alpha.. In one embodiment, the proportionality constant
.alpha. may be approximately equal to one-half. The ridge height H
may be any suitable number. In one embodiment, the ridge height H
is approximately proportionally to the grating period T with a
proportionality constant .beta.. The proportionality constant
.beta. may be an odd integer number times a quarter of the design
filter wavelength. Typical values of the ridge height H may be 35
nm, 105 nm, and 175 nm.
Then, a multi-layer coating 230 is deposited conformally on the
grating structure 220 and the mirror substrate 210. The multi-layer
coating 230 has a number of layers, or stack, of first and second
materials having either high and low atomic numbers, respectively,
or high and low densities of charge carriers, respectively.
In one embodiment, the first material is molybdenum (Mo) and the
second material is silicon (Si) or beryllium (Be). The multi-layer
coating 230 may also have a number of layers of a compound
interspersed within the layers of the first and second materials.
In one embodiment, the compound is silicon carbide (SiC). The
incorporation of the compound SiC in the stack improves heating
durability with minimal reduction (e.g., 3% to 5%) in
reflectivity.
The reflective spectral filter 130 in this embodiment therefore
includes a multi-layer coating 230, the grating structure 220, and
the mirror substrate 210. The multi-layer coating 230 is designed
to be reflective at around the actinic wavelength (e.g., 13.4 nm)
in an optical path and wavelengths longer than the actinic
wavelength (e.g., 60 nm, 140 nm). With this construction,
wavelengths that are both near the grating period T and that
reflect from the multi-layer coating 230 are diffracted away
(diffracted rays 150 shown in FIG. 1) from the path of the actinic
rays (actinic rays 160 shown in FIG. 1).
FIG. 3 is a diagram illustrating a process 300 to fabricate the
reflective spectral filter 130 shown in FIG. 1 according to another
embodiment of the invention.
The process 300 starts with polishing a mirror substrate 310. This
step is much similar to the step shown in FIG. 2. Next, a first
multi-layer coating 320 is deposited on the mirror substrate 310.
The first multi-layer coating 320 is made of materials similar to
the multi-layer coating 230 in FIG. 2. It includes a number of
layers, or stack, of first and second materials having either high
and-low atomic numbers, respectively, or high and low densities of
charge carriers, respectively.
In one embodiment, the first material is molybdenum (Mo) and the
second material is silicon (Si) or beryllium (Be). The multi-layer
coating 230 may also have a number of layers of a compound
interspersed within the layers of the first and second materials.
In one embodiment, the compound is silicon carbide (SiC).
Then, an etch stop layer 330 is optionally deposited on the
multi-layer coating 320. The etch stop layer 330 may be made by SiC
or any other suitable material. The etch stop 330 is used so that
subsequent etching does not cut into the multi-layer coating
320.
Next, a metal spacer layer 340 is deposited on the etch stop layer
330 to provide grating relief layer. Then, a second multi-layer
coating 350 is deposited on the metal spacer layer 340. This second
multi-layer coating 350 is essentially the same as the first
multi-layer coating 320. Both the multi-layer coatings 350 and 320
are designed to be reflective at around the actinic wavelength
(e.g., 13.4 nm). Note that the metal spacer layer 340 may not be
needed. In addition, the second multi-layer coating 350 may not be
needed leaving only the metal spacer layer 340.
The process 300 then lithographically patterns and etches a grating
structure 360 from the second multi-layer coating 350 and the metal
spacer layer 340. The grating structure 360 may have a
one-dimensional layout or a two-dimensional layout. The layout
pattern 370 shows a representative two-dimensional layout having
ridges 372 and grooves 374. The grating structure 360 has a grating
period T responsive to a band around a longer design filter
wavelength (e.g., 60 nm) and selected harmonics. The grating period
T is selected to cause diffracting, out of an optical path, an
incident radiation within this band around of the design filter
wavelength. Finally, the portion of the etch stop layer 330 that is
exposed is removed. Alternatively, the etch stop layer 330 may be
left on the first multi-layer coating if it is thin and
transparent. The grating structure 360 includes a number of ridges
spaced at the grating period T. Similar to the grating structure
220 in FIG. 2, the ridges have a ridge width W and height H. The
ridge width W and height H may be any suitable values. In one
embodiment, the ridge width W is approximately proportionally to
the grating period T with a proportionality constant .alpha.. The
ridge height H is approximately proportionally to the grating
period T with a proportionality constant .beta..
Similar to the embodiment shown in FIG. 2, the reflective spectral
filter 130 in this embodiment therefore includes a first
multi-layer coating 320, the grating structure 360, and the mirror
substrate 310. The grating structure 360 may include a metal spacer
340 only, a second multi-layer coating 350 only, or a combination
of the metal spacer 340 and the second multi-layer coating 350 as
shown. The first and second multi-layer coatings 320 and 340 are
designed to be reflective at around the actinic wavelength (e.g.,
13.4 nm) in an optical path. With this construction, wavelengths
that are both near the grating period T and that reflect from the
multi-layer coatings 320 and 340 are diffracted away (diffracted
rays 150 shown in FIG. 1) from the path of the actinic rays
(actinic rays 160 shown in FIG. 1).
An electromagnetic (EM) simulation is performed to study the
effects of the reflective multi-layer coating. The results of the
simulation are shown in Table 1 and FIG. 4. Table 1 shows
components of the multi-layer coating as used in FIGS. 2 and 3. It
is noted that the materials and thickness are merely for
illustrative purposes.
TABLE 1 Multi-layer coating components Thickness No. Layer (nm) 1
Si 3.48 2 Mo 3.69 3 SiC 3.40 4 Mo 3.60 5 Si 3.41 6 Mo 3.58 7 Si
3.41 8 Mo 3.63 9 SiC 3.46 10 Mo 3.53 11 Si 3.45 12 Mo 3.51 13 Si
3.47 14 Mo 3.59 15 SiC 3.49 16 Mo 3.46 17 Si 3.52 18 Mo 3.44 19 Si
3.54 20 Mo 3.54 21 SiC 3.52 22 Mo 3.38 23 Si 3.59 24 Mo 3.35 25 Si
3.62 26 Mo 3.49 27 SiC 3.56 28 Mo 3.29 29 Si 3.66 30 Mo 3.26 31 Si
3.69 32 Mo 3.45 33 SiC 3.60 34 Mo 3.21 35 Si 3.73 36 Mo 3.17 37 Si
3.77 38 Mo 3.40 39 SiC 3.64 40 Mo 3.13 41 Si 3.80 42 Mo 3.10 43 Si
3.83 44 Mo 3.36 45 SiC 3.67 46 Mo 3.07 47 Si 3.86 48 Mo 3.04 49 Si
3.89 50 Mo 3.33 51 SiC 3.70 52 Mo 3.02 53 Si 3.90 54 Mo 2.99 55 Si
3.93 56 Mo 3.31 57 SiC 3.72 58 Mo 2.98 59 Si 3.93 60 Mo 2.95 61 Si
3.96 62 Mo 3.29 63 SiC 3.74 64 Mo 2.95 65 Si 3.95 66 Mo 2.93 67 Si
3.98 68 Mo 3.28 69 SiC 3.75 70 Mo 2.94 71 Si 3.97 72 Mo 2.91 73 Si
3.99 74 Mo 3.27 75 SiC 3.75 76 Mo 2.92 77 Si 3.98 78 Mo 2.90 79 Si
4.00 80 Mo 3.26 81 SiC 3.76 82 Mo 2.91 83 Si 3.99 84 Mo 2.89 85 Si
4.01 86 Mo 3.26 87 SiC 3.76 88 Mo 2.91 89 Si 3.99 90 Mo 2.89 91 Si
4.01 92 Mo 2.6
FIG. 4 is a diagram illustrating reflectance and absorption spectra
for the multi-layer coatings according to one embodiment of the
invention. The diagram shows high reflectivity at the design
wavelength of 13.4 nm (92.5 eV) and also unwanted wavelengths
longer than 60 nm (less then 20.7 eV).
The diagram shows the reflectance and absorption spectra for a
white (flat) source from a {Si (Mo SiC Mo Si Mo Si) 15 Mo Si}
multi-layer with thickness optimized for a 5-degree incidence. The
total number of layers is 92. The total Mo thickness is 147.97 nm.
The total Si thickness is 117.31 nm. The total number of SiC
thickness is 54.54 nm.
It is clear from the diagram that the reflectance is maximum at two
places: a narrow band around 13.4 nm and a narrow band around 60
nm. The absorption spectrum is correspondingly minimum at these two
bands.
FIG. 5 is a diagram illustrating a system 500 using a transmitting
spectral filter in which one embodiment of the invention can be
practiced. The system 500 includes an EUV radiation source 510, a
condenser mirror 520, a fold mirror 530, and a transmitting
spectral filter 540. The system 500 is typically used in EUV
lithography.
The EUV radiation source 510 and the condenser mirror 520 are the
same as the EUV radiation source 210 and the condenser mirror 220
shown in FIG. 2. The fold mirror 530 is standard mirror that
reflects the incident radiation from the condenser mirror 520. The
fold mirror 530 may have multi-layer coating that reflects a narrow
band around a design wavelength (e.g., 13.4 nm) and wavelengths
longer than a second wavelength (e.g., 60 nm). The reflected
radiation forms an optical path 550.
The transmitting spectral filter 540 is inserted into the optical
path 550. The transmitting spectral filter 540 has a grating
structure or a metal mesh with a low aspect ration supported on an
ultra-thin layer of transmitting material (e.g., Nitride, Oxide).
The desired actinic rays (e.g., the 13.4 nm rays) are much shorter
than the period of the mesh or the grating structure and pass
through the transmitting spectral filter 540 with little blocking.
The rays having longer wavelengths that are near the period of the
mesh or the grating structure are not transmitted and undergo
reflection and diffraction, forming the reflected and diffracted
rays 560.
FIG. 6 is a diagram illustrating a process 600 to fabricate the
transmitting spectral filter 540 shown in FIG. 5 according to one
embodiment of the invention.
The process 600 starts with polishing a wafer blank 610. Then, an
ultra-thin film layer 620 is deposited on the wafer blank 610. The
ultra-thin film layer 620 is made of a transmitting material such
as Nitride or Oxide or any other material that has high
transmittance.
Next, a metal layer 630 is deposited on the ultra-thin film layer
620. Then, the metal layer 630 is lithographically patterned and
etched to become a metal mesh or a grating structure 640. The
grating structure 640 may have a one-dimensional layout or a two
dimensional layout. The grating structure 640 has a grating period
T responsive to rays that have wavelengths longer than a design
wavelength (e.g., 13.4 nm). Specifically, the grating structure 640
has a grating period selected to reflect or diffract incident
radiation having wavelengths longer than the second wavelength
(e.g., 60 nm). The grating structure 640 has a number of ridges
spaced at the grating period T. The ridges have a ridge width W and
height H. The ridge width W and height H may be of any suitable
values to provide the desired characteristics. In one embodiment,
the ridge width W is approximately proportionally to the grating
period T with a proportionality constant of .gamma.. A typical
value of .gamma. is much less than 0.5.
Then, the process 600 lithographically patterns and etches the back
side of the wafer 610 to form a support structure 615. The support
structure 615 provides support or frame for the thin-film layer 620
and the grating structure 640. The layout pattern 660 shows a
representative two-dimensional layout having ridges 662, grooves
664, and support structure 615.
While this invention has been described with reference to
illustrative embodiments, this description is not intended to be
construed in a limiting sense. Various modifications of the
illustrative embodiments, as well as other embodiments of the
invention, which are apparent to persons skilled in the art to
which the invention pertains are deemed to lie within the spirit
and scope of the invention.
* * * * *